I'm with a lot of people here that I originally believed that Sephiroth never thought Cloud as anything special. He messed with Cloud the same way he messed with a lot of his clones, they were inferior, he was going to be a god, so he was going to flaunt his superiority while talking shit.
At least when I first started playing the OG, I always thought that he overlooked Cloud. I think what really made me think that was during that scene in the Shinra Mansion when he's like "ur here, get in loser we're going shopping" like he barely registered Cloud. And when Sephiroth did play with him, it was less because Cloud is special but because he was catching up to Sephiroth and has access to all of the things he needs (black materia, Aerith) while being a puppet.
It sort of went hand in hand with Cloud's delusions; he thought he has this epic personal battle, he thought he was a soldier, he thought he had a cool personality when most of it was in his head. And only as he continued his struggle was he able to be his best.
I think that's what caused me some dissonance between me and AC/C, it made me change my interpretation of Sephiroth. I guess I never thought of his primary motivator as hate. I mean, there was some, and definitely disgust at the wolrd, but imo it was always about him being better than everything. And that translated to Cloud, he didn't hate Cloud, he was just superior to him and it was confusing as shit to him that Cloud was his puppet, a loser, and he still bested him. That look of major confusion at the ending of FFVII was like "That was not as planned."
His focus on Cloud was strange to me, because it was all about Cloud. When in the OG, I always thought his focus on Cloud was less about Cloud and more about Sephiroth's superiority. In AC/C it was about hurting Cloud because he was Cloud, it almost made it feel like he thought of him as an equal or someone worthy of messing with, rather than Sephiroth believes he's so great and everyone around him is so worthless- so why not.
I guess I can see now how Sephiroth enjoyed toying with Cloud in the OG, but even then it was more bitterness about this boy who managed to throw him into the reactor and now he's enjoying conquering him, because like Ryu said- hubris.
So yeah, that's what I thought Sephiroth has been missing in the more recent adaptations, narcissism. I honestly think this was his major fault, how he justified all the horrible things he did, and what caused his downfall, at least I did before. Headcanon I guess.